This invention relates to an electrical contact mat with at least two conductive plates arranged at a spacing from each other by nonconductive spacers in an at-rest condition. The electrical contact mat is particularly useful in safety devices, warning devices and intrusion protection apparatus.
Fabrication of plates of an electrical contact mat from metal is disclosed in a European patent document EP-0109 159 of Minnesota Mining and Manufacturing Company published May 5, 1984. Such contact mats may possibly be suitable for rough service with large switching forces, but they are not advantageous when smaller point loads initiate a contact. What is more, they are not adaptable to uneven surfaces, curvatures and the like.
European patent document EP-0 395 784 of Karlheinz Beckhauser published on Nov. 7, 1990 shows how to fabricate the plates from rubber, plastic and the like and to provide them with spacers. The spacers are designed as bumps, strips or battens and fabricated in one piece with one plate. In this way, plates are obtained that are complicated and thus expensive to fabricate, especially since the plate with spacers must be designed in such a way that the spacers are not conductive but the plate is conductive.
Further, it is generally known to provide an electrical switching mechanism on safety devices in order, for example, to shut off a machine or to actuate a warning device when a switch initiates a switching pulse. So-called closed-circuit protection is carried out in order to detect open circuits, short circuits and the like. Here, besides the switching mechanism, a tactile sensor also has an electrical termination, this electrical termination lying in a closed circuit. Permanently flowing through this electrical termination is a closed-circuit current, which has a constant value if no switching pulse is present. If a short circuit or an open circuit should suddenly occur, it is detected through variation of the constant closed-circuit current. Similarly, the closed-circuit current changes if the switch short-circuits the electrical termination, so that a switching operation can be initiated by this change of known magnitude in the closed-circuit current. Here it is disadvantageous that the current flow through the electrical termination means that a voltage is present across this electrical termination, leading, on the basis of environmental effects prevailing at the place of installation of the electrical termination, to material damage to the components involved and their connection and to further impairment in signal detection. This leads not only to damage to or even failure of the tactile sensor, but also to the necessity of frequent replacement of the sensor, which is undesirable.
Such safety-critical devices, as a rule, are operated in a voltage range that lies below the so-called low-voltage protection value of 42 volts, preferably 12 or 24 volts being employed. Significant material damage phenomena are detectable even at these voltage values, leading to the disadvantages already described, which are most especially safety-critical. Even the use of high-quality materials in the region of the electrical termination, including the electrical connections, has proved not to lead to significant prevention of material damage.
It is an object of the invention to remedy the disadvantages of the contact mats hereinbefore described and to create a contact mat that is simple and inexpensive to fabricate and reacts in a reliable way to small forces. It is a further object of the invention to avoid the disadvantages of the prior devices for closed-circuit protection, such as damages to or failures of the tactile sensor, with inexpensive materials even if the electrical contact mat is used in a moist or wet environment.
According to the invention, the spacers are designed as a substantially inelastic interlayer, which has openings of such size that the inner surfaces of the plates make mutual contact under compressive load and thus form the switch of the tactile sensor of the closed-circuit protection, the plates being designed as thin, preferably pliable and elastic switching elements. The plates preferably have a thickness of 2.5 mm, the non-conductive interlayer, which is preferably designed as a mesh fabric or perforated film of plastic or cotton, having a preferred thickness of 0.5 mm. The openings in an interlayer a mesh fabric preferably have a mesh width of approximately 5 mm. The plates preferably have a Shore hardness of 50 to 65.
If the interlayer, as proposed according to the invention, is made adhesive on both sides, a unitary contact mat can be fabricated with simple resources. An adhesive tape, for example, can be used as edging in order to improve the durability.
It is further proposed, according to the invention, that the plates have bumps or other protrusions arranged at intervals on their surface opposite the interlayer. In this way, the contactability is enhanced, especially if what is applied is more an areal than a point load.
According to the invention, the proposed contact mat is suitable for the formation of a bumper or other safety switching apparatus with large overtravel, force transmission elements then being located next to the contact mat, which force transmission elements are attached to a support element. An elastic superstructure, preferably made of foamed plastic, is provided on the side of the support element opposite the transmission elements. The support element is articulated so that it can move perpendicularly to the contact mat and is made substantially rigid, so that forces on the elastic superstructure lead to the initiation of a contact of the switching mat. On the end toward the contact mat, the transmission elements are made in such a way that they form a small transmission area lying in the range of 10 mm2. If a plurality of transmission elements are arranged, for example, in multiple rows, even an oblique force on the elastic superstructure can lead to reliable initiation of the contact mat. Thus, switching initiation can be varied through the number and configuration of the transmission elements.
In order to insure an articulation of the support element with elastic superstructure, relative to the contact mat, such that the support element can move perpendicularly to the contact mat, the contact mat and the support element can be placed in a frame, the frame enclosing both these elements and holding them against each other with a rim.
The proposed contact mat is also preferably suitable for the formation of a safety edge switch, the contact mat then being made as a contact strip with which a support strip, which is articulated so that it can move perpendicularly to the contact strip, is in contact via transmission elements. The structure is similar to that of the bumper, here the question being not primarily one of large. overtravel but of a particular kind of safety edge switch. The transmission. elements can be fabricated in one piece with the support strip, and the support strip should have a certain flexibility so that contact initiation also takes place as a consequence of a point load.
The proposed contact mat can also be bent into an arch shape and attached at its two ends to a machine, a rolling gate or the like and designed as a safety edge switch. A pressure on the outer jacket of the contact mat bent into an arch shape initiates a switching pulse.
According to the invention, the current flowing in the closed circuit can be varied through loading and unloading of the contact mat, and there are ways of generating the current flowing in the closed circuit such that a voltage of maximally 1 volt is present across the electrical termination.
It has been found that material damages in the region of the tactile sensor, that is, in the region of the contact mat and of the electrical termination including the electrical connections, can be markedly reduced or substantially prevented by working with voltages in the range below 1 volt. The generation of the current flowing in the closed circuit by equipment that causes the voltage applied across the electrical termination to be maximally 1 volt has the advantage that the previous equipment for generating the voltage or the current flowing in the closed circuit can be retained. In particular, it has been found that this voltage in particular makes the electrical contact mat insensitive to moisture and the like.
If the equipment for generating the current flowing in the closed circuit is designed as a:voltage source, then specifying the voltage with allowance for the value of the electrical termination (thus the resistance value if a resistance is used as the electrical termination) sets the current flowing in the closed circuit and simultaneously limits the voltage across the electrical termination to the maximum value of 1 volt.
If the equipment for generating the current flowing in the closed circuit is designed as a current source, which imposes in the electrical termination of the tactile sensor a current from which the voltage of maximally 1 volt results, then the maximum voltage of 1 volt (or a constant value lying below 1 volt) can be maintained by appropriate dimensioning of the imposed current and the value (resistance value) of the electrical termination (resistance).
In development of the invention, a tactile sensor may be connected to an electronic interpretation unit that has at least one trigger as well as one switching stage. In this way, a compact interpretation unit can be achieved, which not only detects an obstruction using the tactile sensor but also uses the trigger for interpretation and uses a switching stage to initiate a switching operation.
In development of the invention, the equipment (voltage source or current source) is arranged internally or externally to the electrical interpretation unit. In this way, the place of installation of the equipment for generating the current flowing in the closed circuit can be freely chosen and adapted to the prevailing design circumstances.